Unlike the right heart cavities, for which it is generally sufficient to implant endocardial leads via the right peripheral venous network, the implantation of permanent leads into a left heart cavity involves substantial surgical risks, in particular the risk of bubbles passing to the cerebral vasculature located downstream of the left ventricle.
One known technique, described for example by the U.S. Pat. Publication No. 2009/0299447 A1, is to apply an epicardial lead electrode against the outer wall of the myocardium, facing the cavity to be stimulated. But to stimulate a left heart cavity, instead of introducing a lead into the cavity directly, the technique that is most often used—and the one that is referred to by the present invention—is to introduce a lead into the coronary network. That lead is provided with an electrode that is to be applied against the wall of the epicardium, and oriented toward the left ventricle or the left atrium, as appropriate. These leads stimulate the heart muscle via one or more electrodes whose positions depends on the predefined trajectory of the cannulated vein.
A lead of this type is, for example, the SITUS LV (trademark) model, available from Sorin CRM (Clamart, France) and described in EP 0993840 A1 and its counterpart U.S. Pat. No. 6,385,492 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical). Also, U.S. Publication No. 200310220677 A1 discloses a lead of the same type.
Such a lead is introduced into the coronary sinus from its opening in the right atrium. The lead is then pushed and oriented along the network of the coronary veins to the selected site. This procedure is very delicate, given the peculiarities of the venous system and its access paths, requiring passage through valves and tortuosities as well as the gradual reduction of the venous diameter as the lead progresses in the selected coronary vein.
Once the target vein is reached, the surgeon looks for a satisfactory pacing site, with good electrical contact of the stimulating electrode against the tissue of the epicardium, this contact having to be maintained despite the various variations or stresses over time.
It has been proposed to have multiple electrodes along the lead body and optionally to give the lead body a particular configuration to increase the chances of an acceptable compromise. [The surgeon can thus select, among the various electrodes present on the lead body, the one(s) providing the best efficiency from both the electrical and hemodynamic points of view. Such a multi-electrode lead is described, for example, in EP 1938861 A1 and its counterpart U.S. Pat. Publication No. 2008/0177343 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical). These leads allow the implementation of a concept known as “electronic repositioning,” aimed at directing or redirecting the electric field between different electrodes arranged along the pacing lead of the left ventricle and/or with one of the electrodes of the pacing lead of the right ventricle. This technology allows the management of micro-movements or changes in the hemodynamic behaviour (e.g., reverse modeling), simply by reprogramming the generator via telemetry through the patient's skin, without requiring any significant surgical intervention.
An alternative to this solution is an increasing complexity of the structure of the lead. For example, increasing the number of electrodes causes an increase in the number of components, and therefore of electrical connections. Or it requires the use of multiplexing circuits for the selection of the various electrodes present on the lead. However, these alternatives result in an increased risk of mechanical failure.
U.S. Pat. Publication No. 2009/157136 A1 describes a technique for finding an optimal pacing site using a temporary mapping catheter to be introduced into the coronary sinus. This catheter is either a flexible tube open at both ends, or a guidewire. In either case, it includes multiple electrically independent distal electrodes, and in the proximal portion a connector for connecting to an acquisition system for identification of the best stimulation site using an algorithm based on the cardiac motion. A classic permanent definitive multielectrode lead of standard diameter from 4.5 to 6 French (1.5 to 2 mm) is then placed in the selected position, by use of an over the wire (OTW) technique in the case a guide wire is used, or through a lumen of a temporary introducer catheter if one is used.
Another recent development in the field of pacing the left ventricle is the reduction of the diameter of the portion implanted in the coronary network, to about a diameter of 4 French (1.33 mm). The size of the lead body is a factor directly related to the ability to control and guide the lead into and through the coronary venous system, in order to be able to select specific stimulation sites located in certain collateral veins. These sites are typically reached by use of a vein sub-selection catheter for the introduction of a guiding stylet to the chosen site. Once the vein is selected and stylet is introduced, the surgeon then advances the lead body by sliding it over the stylet, the latter acting as a support guide wire of small diameter, axially guiding the lead body until it reaches the selected location (i.e., the OTW technique).
These foregoing options, however, have recognized the existence of two competing limitations, which are:                The thinness of the lead, whose diameter determines whether or not allow it can reach the deepest collateral veins: thus, for the aforementioned SITUS LV lead, the lead has a diameter of 6.6 French (2.2 mm) and requires an introducer of 7 French (2.33 mm) in diameter, and        The correct positioning and maintaining the good electrical contact of the electrode against the tissue for stimulation of wall of the epicardium.        
The solutions proposed so far have been a compromise between these two constraints. For example, whereas the above techniques of using a multi-electrode lead or electronics repositioning allows, more or less, to overcome the second limitation, they exacerbate the first limitation because the multiplication of electrodes or internal conductors and components necessarily implies an increase in the diameter of the lead body and a reduction in its flexibility, making it difficult or impossible to pass through the tortuosities.
The present invention aims to overcome these two limitations, by providing a pacing lead for the left ventricle (and alternatively for the atrium) whose active part:                Has a very small diameter, to exploit the full length of the vein and to make optimal use of all the veins present in the basal zone;        Ensures excellent electrical contact with the tissues to be stimulated; and        Importantly, increases or expands the areas of stimulation, simultaneously allowing (unlike traditional leads) to stimulate several areas in the epicardium, thereby improving the chances of an optimal cardial resynchronization therapy (CRT).        
In regard of the latter point, it has been found that multiple stimulation points on the left ventricle are a factor to improve substantially the quality of CRT.
Current studies nevertheless show that it is very difficult to concurrently implant two leads in the coronary venous system. A known alternative is to equip a lead body with several electrodes, up to four electrodes in some models. However, these leads have a relatively large diameter of about 4 French (1.33 mm), due to the complexity of components and connections necessary for the selection of electrodes for electronic repositioning. Furthermore, the relative position of the selected pairs of electrodes is very limited, these pairs of electrodes also being necessarily positioned in the same coronary vein, usually the posterolateral vein.